Generally, in X-ray photographic silver halide light-sensitive materials applicable to medical diagnoses or the like (hereinafter referred to as X-ray light-sensitive materials), a high-speed emulsion containing silver halide grains having a large size of several microns are coated on a colored support or base. Such large-sized grains run short of the covering power of developed silver. The group of coexisting small-sized grains deviates from a sensitive region and not only contributes nothing to any optical density but makes the optical density lower because of the shortages of light-receiving quantity and sensitizing effect.
It is, therefore, usual that an amount of silver coated is increased and the both sides of a support are coated with.
Medical diagnoses require an X-ray light-sensitive material having the characteristic curve suitable for expressing the soft tissues such as the stomach and intestines or the bone tissues such as those of the hand and legs.
It is, however, not only difficult to satisfactorily express both soft and hard tones, but also lower in the efficiency of silver halide application, when using a single emulsion.
In some practical X-ray photographing examples prepared with conventional type direct radiographic light-sensitive materials, there have been some instances where even the following serious defects or inconveniences are found. To be more concrete, one of the most popular in-vivo organism sites subject to X-ray photographing is the chest. When reading chest X-ray photographs, the important sites are the blood vessels of the lung fields and the coronaries behind the heart.
Such lung fields are in a moderate density region (having a density of D=1.3 to 1.5). For reading the images of the blood vessels in the lung field, a relatively higher sharpness is required. At the same time, for reading the images of the coronaries, a wide latitude is required, because such coronary images are in a relatively lower density region (having a density of D=0.05 to 0.3).
With conventional high-gamma types of X-ray light-sensitive materials, the sharpness of a lung field image may be expressed high, while the image density of the coronary arteries may be expressed extremely low. Therefore, those light-sensitive materials have been practically unable to contribute to diagnoses. On the contrary, when using a low-gamma type X-ray light-sensitive material, the image of the coronary arteries has been able to express well, while a lung field image has been low in sharpness.
As the methods of forming an X-ray image capable of satisfying the above-mentioned two contradictory requirements, sensitivity-compensation type intensifying screens were studied and they are disclosed in, for example, Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) Nos. 62-138800(1987), 62-170900(1987), 62-222200(1987), 62-231199(1987), 62-231200(1987), 62-238500(1987), 63-186200(1988), 63-223600(1988), 63-233340(1988) and 63-233399(1988).
However, these methods have been unable to compensate the differences between individuals and still not satisfactory to form an image suitable for efficient diagnoses.
On the other hand, the studies have been progressed from the aspect of controlling an X-ray dose. From these studies, an X-ray masking method or a scanning method using an X-ray pencil-beam spot was proposed and, further, a method using a scanning X-ray fan-beam by which the defects of X-ray pencil beams are improved was proposed. (Refer to U.S. Pat. No. 4,433,430.) However, in the X-ray photography using such a scanning fan-beam, the fan-beam fluxes have each the same intensity in the transverse direction when they cross through each of the thick, thin, hard and soft tissue sites of a subject, so that the details of an image in the transverse direction could not satisfactorily be expressed. For improving the above-mentioned problem, Japanese Patent O.P.I. Publication No. 62-129034(1987) proposed a divergent X-ray radiographic system (hereinafter referred to as an X-ray fan-flux regression scanning system) such as a scanning X-ray fan-beam flux system in which the fan-beam flux is diversified into a plurality of fan-beam fluxes and each individual flux is modulated separately by a feed-back system.
The above-mentioned X-ray photography using the X-ray fan-flux regression scanning system is very valuable, because it is capable of solving the differences between individual subjects and the obstacles of transversely positioned hard and soft tissues with the use of a simple X-ray film screen cassette so as to form excellent images if using an X-ray film having a gamma of not lower than 3. However, this method is still not satisfactory to express soft tissues coronary arteries covered by such soft tissues as. It has not, therefore, been achieved yet to provide any delicate and accurate diagnosis information.